Two important specifications of the isopropyl alcohol, or any other cleaning agent you may choose, are 1) the purity specifications, and 2) the filtered specifications. When dealing with the precision of optics and the need for eliminating as many contaminants as possible that can cause damage or block signals, the specifications are very important. You need to look for the best available, and that is 99% isopropyl alcohol with a filter spec of 0.2 micron. Any less concentration than 99% will add to the slower evaporation and water spotting of the optical surface. Using the best filter grade of alcohol will ensure a surface that has the least possibility of having other particular contaminants to block or reduce signals or cause damage through scratches and burning. Note: Specs of over 99% will not make a difference since 100% is only possible through very controlled conditions and fractions of percentages over 99% look good but are likely to not be realized as soon as the product is opened in a non-controlled (real world) environment due to absorption of humidity. Also, the bottle may say 99%, but if there are no grade specifications you can not be sure what you have.